The present invention relates to circuit arrangements for the multiplexing of signal functions at a single terminal of an integrated circuit (IC), and more particularly, to switchably coupling an extrinsic signal to the IC so that IC operational functions are available to control the extrinsic signal.
With much signal processing circuitry manufactured in integrated circuit form, it is desirable for the IC manufacturer to minimize the number of connections made to the integrated circuits. This is because connections to external circuits must be done through pins on the integrated circuit package, which are in part, a determinant of the size and cost of the integrated circuit.
In the instant case it is desirable in a low cost electronic device, e.g., a television receiver or radio receiver, to provide front panel audio/visual input jacks in a device having an internal volume control in an IC manufactured for processing an audio signal with no provision made for an external audio input. To incorporate such a desirable feature with such an IC would ordinarily require using a separate audio IC in order to provide a volume control for such an auxiliary audio signal. Such an approach would incur a considerable extra cost. Thus, to make maximum use of such an IC, it is desirable to combine or multiplex additional functions on or at a terminal/pin of an IC which had been previously dedicated by the IC manufacturer for other purposes. As used herein, the terms pin and terminal for an IC are synonymous.
The multiple use of an integrated circuit pin is shown in U.S. Pat. No. 4,434,474 of Best et al., U.S. Pat. No. 4,293,870 of Steckler et al., of an antecessor of the assignee of the present patent application, and U.S. Pat. No. 4,173,769 of van Straaten. U.S. Pat. No. 5,594,442 of Paulos et al. shows a digital IC wherein connection of an external digital signal is made to an IC pin used for switching a digital de-emphasis circuit. U.S. Pat. No. 4,675,550 of Hafner shows using a single input terminal for two signals having differing voltage levels.
In a signal processing system with an IC having an intrinsic signal provided at a pin of the IC, a first operational function, for the signal is provided at the pin, and further along in the signal flow path, a second operational function, for the signal is provided within the IC. An extrinsic signal is switchably coupled to the pin so that the second operational function can be used to operate on the extrinsic signal. The second signal is coupled to the pin at a low source impedance so that when the second signal is switched to be operational, the first operational function is defeated, and the first signal is severely attenuated. When the circuit is switched to not couple the second signal to the pin, the signal path for the extrinsic signal and the low source impedance are both removed, thus restoring the first operational function and the first signal at the pin of the IC.